1. Field of Invention
The present invention relates to a resource scheduling method for wireless network, and in particular to a resource scheduling method for multi-hop relay broadband wireless network.
2. Description of Prior Art
Broadband wireless communication technology and network have been in a rapid progress in the past few years, and these emerging wireless communication systems each have higher operating carrier frequencies due to the planning of frequency spectrum. The existing wireless access network architecture is basically a single-hop network composed of BS (Base Station) and MSS (Mobile Subscriber Station), and each MSS carries out network access directly served by a BS. Such single-hop network has difficulty in meeting the requirement of a high-frequency wireless communication system, since high-frequency signals are poor in transmittance and diffraction and thus vulnerable to Shadow Fading Effect caused by landform. Further, electromagnetic wave undergoes larger attenuation as the band becomes higher, thereby leading to a reduced coverage of base station. For example, according to the result of practical network planning, with a higher transmission frequency being employed, a base station of WiMAX (Worldwide Interoperability for Microwave Access) has an effective coverage radius of only about a few kilometers within urban district, and the effective data transmission rate decreases drastically as the distance between MSS and BS increases. In other words, BS of such high-frequency wireless communication system using single-hop network has a small coverage, and a user located on the edge of BS coverage cannot obtain the guarantee of higher data transmission rate and QoS (Quality of Service).
An approach to solve the above problems is repartitioning each coverage so that it is reduced to some extent that each BS can ensure a high QoS. In this way, the number of BSs must be expanded to bring respective regions into each coverage. This method is inconvenient in terms of the difficulty and cost of BS construction. Thus, for wireless communication network, the concept of multi-hop network is proposed, which forwards signals between BS and MSS by use of one or more tiers of Relay Stations (RSs). Since a node in the multi-hop network can reach its adjacent node with a lower transmitting power, the transmitting power of RS can be reduced dramatically. Besides, a plurality of RSs can be dispersed in terms of location, which helps to effectively avoid the influence of landform on network layout.
Unfortunately, it is necessary to relay duplicated data between BS and RSs in the multi-hop relay network, resulting in waste of system resource in terms of signal transmission time overhead or frequency occupation and thus a shrunk system capacity. For this reason, resource reuse technology is necessary. Here, resource can be time in a TDMA (Time Division Multiple Access) communication system, frequency bandwidth in a FDMA (Frequency Division Multiple Access) communication system, pseudo-random code in a CDMA (Code Division Multiple Access) communication system or any other type of resource in other multiple access multiplex communication systems. Different types of resource do not cause any variation in practical application. For the purpose of description, hereafter “resource requirement” represents the requirement quantity of a particular resource type in each of different multiple access multiplex communication systems, “resource scheduling” refers to the resource allocation of BS to respective RSs and MSSs directly coupled to the BS itself, and “resource reuse scheduling” means that BS allocates the same resource to different RSs.
FIG. 1 shows the current wireless network topology structure for resource reuse scheduling method in a wireless communication system. This scheme adopts Manhattan model, in which each cell has a symmetric network structure formed of one BS and four RSs, and the inter-cell network layout are identical in structure. As shown in FIG. 1, BS 5 is located among a group of buildings 6 and has 4 surrounding RSs of RS1, RS2, RS3 and RS4 with reference numbers of 1, 2, 3 and 4 respectively. With such architecture, no interference exists between RS1 and RS2 thanks to the blocking role of the buildings, and thus resource reuse scheduling can be applied for the two RSs. This is also true for RS3 and RS4.
Take the TDMA system as an example, one complete time frame in wireless communication has a structure as shown in FIG. 2. The area filled with left diagonals at the head of the time frame represents the time for which BS serves RS 1, 2, 3 and 4, respectively. The middle area filled with right diagonals represents the time for which RS1 and RS2 serve MSS simultaneously, and the middle area filled with squares represents the time for which RS3 and RS4 serve MSS simultaneously. The area filled with vertical lines at the end denotes the time for which BS serves MSS. With such resource reuse scheduling method, it is possible to shorten the length of time frame and conserve system resource.
Since the method gives no consideration to the variation in bandwidth requirement of these RSs, it allocates each RS with the same share of resources. During the simultaneous scheduling of the system on a series of RSs having no interference with each other, because of the difference in required time resource, the scheduling on some RSs has already been fulfilled, while the scheduling on some other RSs continues, and the scheduling on the next series of RSs cannot be started. Therefore, the waste of system resource remains, further leading to the influence on the bandwidth actually obtained by each MSS as well as the difficulty in meeting QoS requirement on each link. Furthermore, this resource scheduling method can be applied to only a RS arrangement of symmetric and fixed locations without taking into account the RS layout of asymmetry and mobility, and thus the application field is limited.